This is down to a bug in a program on your computer that customizes the way windows displays thumbnails. The simplest solution is to drag and drop the picture you want into Portrait Professional instead of using the start button.
Another solution is to run this program which will stop Portrait Professional using windows thumbnails: pp6NoThumbs.exe.
The reason this only seems to happen in Portrait Professional is that when our application displays the file open dialog we automatically select the windows thumbnail view, unlike most other applications.
We do this only to help you to find the picture you want.